Batteries

“Batteries are the most dramatic objects; other things stop working or they break, but batteries… they die.” – Demetri Martin

What are batteries?

Batteries are units containing various combinations of metals and chemicals, between which occur reversible chemical reactions to store and then provide electrical power on demand. Some of the metals and chemicals can be quite nasty and toxic (e.g. cadmium).

Alessandro Volta (who gave his name to the volt) invented the first electric battery in 1800. His plates were of copper and zinc, and the electrolyte was brine. The most common battery chemistries are lead-acid and lithium-ion.

A single 2-volt, lead-acid traction cell, as used in forklifts (a 24-volt forklift would use 12 of these, and a 48-volt forklift, 24).

Lead-acid batteries

Lead-acid cells are built to many differing formats, are commonly used in industry and form the heart of many off-grid, or hybrid renewable energy systems. The lead-acid battery is still popular, because of its affordability, availability, and the fact that it is so robust if looked after properly.

A basic lead-acid battery comprises 3 elements – a collection of positive plates, a collection of negative plates and a liquid or gel electrolyte. The active material of the plates is lead oxide, and the electrolyte is dilute sulphuric acid. In a discharge situation (when giving out energy), elements of the electrolyte react chemically with elements of the plates to release energy in the form of electrons. The flow of electrons through a conductor is electricity. When being charged, the reaction is reversed and the battery absorbs electrons.

Testing the voltage of batteries with a voltmeter.

There are 3 kinds of lead-acid battery:

• gel – the electrolyte is in a gel form. They are used mainly for emergency standby, and are no good in a cycling (charge/discharge) situation.

• glass mat – the electrolyte is held within a soaked glass mat between the plates. They are better than gel batteries for cycling, but over time, anomalies will develop between the individual cells as regards voltage and state of charge. They are used in situations where a liquid electrolyte might spill.

• flooded battery – contains liquid electrolyte that can spill; this is the best type for a renewable energy system. They need to be topped up with distilled water from time to time.

The best types for renewable energy systems are leisure/deep-cycle batteries (for caravans, boats or mobile homes) or traction batteries (forklifts), as they can be repeatedly discharged and recharged. Car batteries are starter batteries, providing short bursts to start an engine, and are unsuitable for home power systems as they rapidly fail in charge/discharge cycles.

Fast charging and discharging reduces battery life so for example, discharging a lead-acid battery in less than 20 hours is undesirable. Also, lead-acid batteries shouldn’t be discharged to below 50% of their capacity and the electrolyte has to be kept at the right level to keep the plates covered.

Forklift lead-acid batteries: investigation, renovation and equalisation.

Lithium-ion batteries

Lithium-ion batteries are more expensive, but are becoming more popular, as they are used in electric vehicles, and in ‘power walls’ (units, about the size of large storage heaters, comprising a lithium battery pack, a charger, an inverter and some software to control it), that can be added to a home power system. Power is stored during the daytime, and then used in conjunction with a power-sharing inverter to provide electricity to the home in the evening (lead-acid batteries can be used in the same way, but you’d have to set up a separate charger and inverter yourself. In a power wall, it’s all there already).

The two main benefits that lithium-ion has over lead-acid is the huge improvement in energy density (batteries are much smaller than lead-acid for a given amp-hour rating); and the lack of any maintenance requirements. This means that the owners do not engage with the technology, which is good for some people, but might be frustrating for more electrically-experienced folk.

Lithium-ion battery.

This form of power wall storage is still prohibitively expensive and has a huge payback time, but as electric vehicles develop then availability and cost of lithium-ion batteries will improve, as second-hand units become available – although you’ll have to set up your own inverter, charger etc.

Lithium-ion batteries must not go above or below set voltage parameters and so a BMS (battery management system) is needed. Some batteries have this fitted in the top of the cell. This means that if the volts go below or above the pre-set, the BMS switches off the cell output (and all the power).

Inverters and backup systems have to be compatible with the different charge voltages needed for lithium-ion batteries.

Battery abuse and how to avoid it.

What are the benefits of batteries?

The main benefit of batteries is for people who wish to generate and use their own electrical power whether on or off grid. If you’re building a renewable electricity system, you’re not looking to use electricity as it’s generated – the sun won’t be shining all the time, and the wind won’t be blowing all the time (although if you’ve got micro-hydro, your stream is probably going to be flowing all the time), and you’re going to want more electricity at some times than others. So you’ll need to store your electricity.

If you’re not on the grid, you’re going to need batteries. There’s a strong environmental case for saying that if you’re in an area where you can have a grid connection, then you should have one, and use the grid like an enormous battery, as the infrastructure is already there. Also, batteries contain lots of noxious metals and chemicals – so the fewer of them the better.

A collection of 600 amp-hour, 2-volt cells wired in series (a 48-volt battery pack); 600 x 48 = 29kWh, and so the available power is 50% of this (you should never discharge a battery to less than 50% of its capacity), or 14.5kWh. Voltage is the electrical pressure and amps is a measure of flow. Amp-hours is flow over time and so can be used to give the capacity of a battery.

A second benefit, however, is not environmental, it’s about energy independence. Many people want to be sure that they are in control of their own energy supply all the time. They don’t want to involve themselves with giant electricity companies, or risk everything going down if there is a power cut, especially as they have generated their portion of the grid electricity themselves. There’s also a case for saying that with loving care, renewable electricity enthusiasts can be responsible for extending the life of second-hand batteries that would otherwise have died in scrap yards.

The third benefit of batteries is transportability – rechargeable leisure batteries allow the use of renewables in boats and vehicles.

Exposing the elements of a lead-acid battery.

What can I do?

First, do your sums – work out how much storage you need. Battery capacity is based on amp-hours and volts. Here’s an example that will (hopefully) make this a bit clearer: a 12-volt bulb drawing 1 amp from a 12-volt battery will draw 1 amp-hour per hour. So if you have a 50 amp-hour battery, then theoretically you should be able to run the bulb for 50 hours before the battery is completely flat. But – you should not discharge a battery below 50% of its capacity, or you will create serious imbalances within the battery over a relatively short period of time. So, in this case – a 12-volt bulb drawing 1 amp – you can safely run for 25 hours with a fully-charged battery.

How to cope with battery terminal corrosion.

If you want to run something bigger – say a fridge – you have to think about wattage. Let’s say the fridge takes 150 watts to run, and the time it is actually running over the course of a day is 3 hours, then it’s using 450 watt-hours (150×3) per day. If you disregard the extra current needed to start the fridge, and the inefficiencies of your inverter, then just to run the fridge, you will need almost 40 amp-hours per day (450/12 – because watts = volts x amps). But when you take extra current and inefficiency into consideration, it will be more like 70 amp-hours. And then, bearing in mind that batteries are only around 80% efficient, you’ll need to put around 80 amp-hours into your battery on average every 24 hours to run the fridge. You can extrapolate from this for the other appliances you want to run.

4 leisure batteries and an inverter on a vehicle with 12 x 80-watt PV panels on the roof, plus a 350-watt wind turbine.

Next get your batteries – from industrial battery manufacturers, or forklift/traction battery suppliers and recyclers. Traction batteries are designed to be cycled, and are much tougher than leisure batteries. You can get them from scrap yards, but only if you really know what you’re doing – you need time and perseverance. The price will vary with the scrap price of lead. The more you understand batteries, the cheaper you can get them. How many? Well, if we say an average house requires about 3.5kW-hours a day, this equates to 150 amp-hours a day from a 24-volt battery bank. So to give enough power for 2 days without discharging the battery below 50%, you would need 600 amp-hours. This represents 2 battery packs of 6 cells per pack. That’s 12 cells, each weighing around 25kg, so you’ll need a bit of space. Batteries can be dangerous, so make sure their box/housing is kept locked and vents to the outdoors. NB: to reliably run a system using 3.5 kWh per day the generation side needs to be large enough, say something no less than 3kW of installed capacity and with a good automatic backup charging system.

Testing the specific gravity of the electrolyte in a cell with a hydrometer.

You’ll need a charge controller in your system to make sure you don’t overcharge and damage your batteries, and to run standard 240-volt appliances from a 12-, 24- or 48-volt battery bank, you’ll need an inverter. It is possible to buy 12-volt appliances, but they are a lot more expensive, plus mainstream 240-volt appliances are being made much more efficient all the time – so it may be as well to focus on getting a really good inverter.

So, looking after batteries is a bit of a dark art, and can be quite complicated. Our publication Wind & Solar Electricity is a good place to start. Batteries are the recalcitrant teenager of your renewables system. Keep an eye on them and they’ll be just grand, but ignore them and they’ll cause problems.

Thanks to Andy Reynolds of the Ecolodge for information.

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Andy Reynolds is a carpenter / joiner and woodsman who has tutored courses and authored books with us. He has lived on a smallholding in Lincolnshire since the early 80s, renovated a house, built a holiday cottage and got off-grid. He records his adventures with educational videos on his YouTube channel.